image quality

Definition: Assessment or subjective measure of how accurately or fully an image of a subject represents that subject. * For a critical observer, it is based on e.g. the brightness and evenness of illumination, contrast, resolution, geometry, colour fidelity and colour discrimination of an observed image. * It can be affected by e.g. lens aberrations, diffraction and reflection effects, pollutants such as dust and scratches on the lens and in the atmosphere, effects of heat on detectors, motion of subject or optical system.
Crucially inexact
For such a vital subject as image quality, it is curious there is no universally applicable precise way of measuring it. But there can be none: quality is just too slippery. The most objective measure is MTF which gives a good measure of the sharpness (contrast related to resolution) of an image but ignores geometry of the image, cannot fully account for many other factors -- and tests only in white light.

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